- J i~ -i

Evolutionary biology is a field plagued by more misunderstanding and miSt~resentation than practically an~r other in science. Adaptive evolution by natural selection is a s i m p l e ~ t , yet the ways in which selection operates are exceedingly subtle and the patterns it generates are ~ t o misinterpret. Especially

in India, the meagre treatment that evolution gets in biology curricula is rife With misinterpretations and

over-simplifications; it is also shockingly out of date by over half a century. Stephen Jay Gould wr~ote extensively and elegantly about misunderstandings of pattern and process in �9 In ~ ~s~ay

reproduced here, Wh:!r appears as Chapter 11 in ' B u f i y f o e S r o n t o s a u r u s ' , he tak~ up t ~ ! ~ . ' ~ f ' t h e evolution of mode/'/1: liorses, a tale that is familiar tO most Of as from high school biolO~ ~ L ~ Gould

shows in his inimitable style how the prejudiced notion of evolution leading tO"some kind of clear

progression up a ladder of increasing perfection was projected onto the dataon fossil horses, leading to

figures reproduced in biology texts world-wide as canonical examplesdf.iidaptive evolution that are,

nevertheless, plain wrong. .- ...... -~

�9 . - A m i t a b h 3oshi

Life's Little Joke Stephen Jay Gould

I STILL DON'T UNDERSTAND why a raven is like a writing desk, but I do know what

binds Hernando Cort6s and Thomas Henry Huxley together.

On February 18, 1519, Cort6s set sail for Mexico with about 600 men and, perhaps more

important, 16 horses. Two years later, the Aztec capital of Tenochtitl~in lay in ruins, and

one of the world's great civilizations had perished.

Cort6s's victory has always seemed puzzling, even to historians of an earlier age who did

not doubt the intrinsic superiority of Spanish blood and Christian convictions. William

H Prescott, master of this tradition, continually emphasizes Cort6s's diplomatic skill in

making alliances to divide and conquer - and his good fortune in despoiling Mexico

during a period of marked internal dissension among the Aztecs and their vassals.

(Prescott published his History of the Conquest of Mexico in 1843; it remains among the

most exciting and literate books ever written.)

Prescott also recognized Cort6s's two "obvious advantages on the score of weapons" - one

inanimate and one animate. A gun is formidable enough against an obsidian blade, but

RESONANCE J November 2002 87

consider the additional impact of surprise when your opponent has never seen a firearm.

Cort6s's cavalry, a mere handful of horses and their riders, caused even more terror and

despair, for the Aztecs, as Prescott wrote,

had no large domesticated animals, and were unacquainted with any beast of burden. Their

imaginations were bewildered when they beheld the strange apparition of the horse and his rider

moving in unison and obedient to one impulse, as if possessed of a common nature; and as they saw

the terrible animal, with "his, neck clothed in thunder," bearing down their squadrons and

trampling them in the dust, no wonder they should have regarded him with the mysterious terror felt

for a supernatural being.

On the same date, February 18, in 1870, Thomas Henry Huxley gave his annual address

as president of the Geological Society of London and staked his celebrated claim that

Darwin's ideal evidence for evolution had finally been uncovered in the fossil record of

horses - a sequence of continuous transformation, properly arrayed in temporal order:

It is easy to accumulate 'probabilities - hard to make out some particular case, in such a way that

it will stand rigorous criticism. After much search, however, I think that such a case is to be made

out in favor of the pedigree of horses.

Huxley delineated the famous trends to fewer toes and higher-crowned teeth that we all

recognize in this enduring classic among evolutionary case histories. Huxley viewed this

lineage as a European affair, proceeding from fully three-toed, Anchitherium, to, Hipparion

with side toes "reduced to mere dew-claws [that] do not touch the ground," to modern

Equus, where, "finally, the crowns of t.he grinding-teeth become longer .... The phalanges

of the two outer toes in each foot disappear, their metacarpal and metatarsal bones being

left as the 'splints'."

In Cat's Cradle, Kurt Vonnegut speaks of the subtle ties that can bind people across worlds

and centuries into aggregations forged by commonalities so strange that they must be

meaningful. Cort6s and Huxley must belong to the same karass (Vonnegut's excellent

word for these associations) - for they both, on the same date, unfairly debased America

with the noblest of animals. Huxley was wrong and Cort6s, by consequence, was ever so

lucky.

Horses evolved in America, through a continuity that extends unbroken across 60 million

years. Several times during this history, different branches migrated to Europe, where

Huxley arranged three (and later four) separate incursions as a false continuity. But horses

88 RESONANCE I November 2002

Cl;,xI'r.5" ,TC

then died in America at the dawn of human history in our hemisphere, leaving the last

European migration as a source of recolonization by conquest. Huxley's error became

Montezuma's sorrow, as an animal more American than Babe Ruth or apple pie came

home to destroy her greatest civilization. (Montezuma's revenge would come later, and by

another route.)

During our centennial year of 1876, Huxley visited America to deliver the principal

address for the founding of Johns Hopkins University. He stopped first at Yale to consult

the eminent paleontologist Othniel C Marsh. Marsh, ever gracious, offered Huxley an

architectural tour of the campus, but Huxley had come for a purpose and would not be

delayed. He pointed to the buildings and said to Marsh: "Show me what you have got

inside them; I can see plenty of bricks and mortar in my own country." Huxley was neither

philistine nor troglodyte; he was simply eager to study some particular fossils: Marsh's

collection of horses.

Two years earlier, Marsh had published his phylogeny of American horses and identified

our continent as the center stage, while relegating Huxley's European sequence to a

periphery of discontinuous migration. Marsh began with a veiled and modest criticism

(American Journal of Science, 1874):

Huxley has traced succesfully the later genealogy of the horse through European extinct forms, but

the line in America was probably a more direct one, and the record is more complete.

Later, he stated more baldly (p.258): "The line of descent appears to have been direct, and

the remains now known supply every important intermediate form."

Marsh had assembled an immense collection from the American West (prompted largely

by a race for priority in his bitter feud with Edwin D Cope - see Essay 5 for another

consequence of this feud!). For every query, every objection that Huxley raised, Marsh

produced a specimen. Leonard Huxley describes the scene in his biography of his father:

At each inquiry, whether he had a specimen to illustrate such and such a point or to exemplify a

transition from earlier and less specialized forms to later and more specialized ones, Professor

Marsh would simply turn to his assistant and bid him fetch box number so and so, until Huxley

turned upon him and said, "I believe you are a magician; whatever I want, you just conjure it up."

Years before, T H Huxley had coined a motto; now he meant to live by it: "Sit down before

fact as a little child, be prepared to give up every preconceived notion." He capitulated to

Marsh's theory of an American venue. Marsh, with growing pleasure and retreating

RESONANCE [ November 2002 89

C2;//'eSeSJ'd'e5

modesty, reported his impression of personal triumph:

He [Huxley] then informed me that this was new to him, and that my facts demonstrated the

evolution of the horse beyond question, and for the first time indicated the direct line of descent of an

existing animal. With the generosity of true greatness, he gave up his own opinions in the face of

new truth and took my conclusions.

A few days later, Huxley was, if anyting, more convinced. He wrote to Marsh from

Newport, his next stop: "The more I think of it the more clear it is that your great work is

the settlement of the pedigree of the horse." But Huxley was scheduled to lecture on the

evolution of horses less than amonth later in New York. As he traveled about eastern

America, Huxley rewrote his lecture from scratch. He also enlisted Marsh's aid in

preparing a chart that would show the new evidence to his New York audience in pictorial

form. Marsh responded with one of the most famous illustrations in the history of

paleontology - the first pictorial pedigree of the horse.

Scholars are trained to analyze words. But primates are visual animals, and the key to

concepts and their history often lies in iconography. Scientific illustrations are not frills

or summaries; they are foci for modes of thought. The evolution of horse - both in

textbook charts and museum exhibits - has a standard iconography. Marsh began this

traditional display in his illustration for Huxley. In so doing he also initiated an error that

captures pictorially the most common of all misconceptions about the shape and pattern

of evolutionary change.

Errors in science are diverse enough to demand a taxonomy of categories. Some make me

angry, particularly those that arise from social prejudice, masquerade as objectively

determined truth, and directly limit the lives of those caught in their thrall (scientific

justifications for racism and sexism, as obvious examples). Others make me sad because

honest effort ran headlong into unresolvable complexities of nature. Still others, as errors

of logic that should not have occurred, bloat my already extended ego when I discover

them. But I reserve a special place in perverse affection for a small class of precious ironies

- errors that pass nature through a filter of expectation and reach a particular conclusion

only because nature really works in precisely and unlikely, but bear with me for the

premier example of life's little joke - as displayed in conventional iconography (and

interpretation) for the most famous case study of all, the evolution of the horse.

In his original 1874 article, Marsh recognized the three trends that define our traditional

view of old dobbin's genealogy: increase in size, decrease in the number of toes (with the

90 RESONANCE t November 2002

C ?[rYr$#C3

~ K C ~ N T .

[ 4 ~ r

EOP.JL~ 1r

O R Ol1=1r pIP'~llt b

g �9 ~

OX,~E&LOGY OF TIL~ HORSR.

The celebrated origi- nal figure drawn by 0 C Marsh for T H Huxley's New York lec. ture on the evolution of horses. This ver- sion appeared in an article by Marsh in the American Journal of Sciencefor1879. NEG. NO. 123823. Courtesy Department of Library Services, Amer ican Museum of Natural History.

hoof of modern horses made from a single digit, surrounded by two vestigial splints as

remnants of side toes), and increase in the height and complexity of grinding teeth. (I am

not treating the adaptive significance of these changes here, but wish to record the

conventional explanation for the major environmental impetus behind trends in locomo-

tion and dentition: a shift from browsing on lush lowland vegetation to grazing of newly l

RESONANCE t November 2002 91

C /T332C Y

evolved grasses upon drier plains. Tough grasses with less food value require considerably

more dental effort.)

Marsh's famous chart, drawn for Huxley, depicts these trends as an ascending series - a

ladder of uninterrupted progress toward one toe and tall, corrugated teeth (by scaling all

his specimens to the same size, Marsh does not show the third "classic" trend toward

increasing bulk).

We are all familiar with this traditional picture - the parade of horses from little eohippus

(properly called Hyracotherium), with four toes in front and three behind, to Man o' War.

(Hyracotherium is always described as ``fox terrier" in size. Such traditions disturb and

captivate me. I know nothing about fox terriers but have dutifully copied this description.

I wonder who said it first, and why this simile has become so canonical. I also wonder what

the textbook tradition of endless and thoughtless copying has done to retard the spread of

original ideas.*)

In conventional charts and museum displays, the evolution of horses looks like a line of

schoolchildren all pointed in one direction and arrayed in what my primary-school drill

Most widely reproduced of al l i l lustrations showing the evolution of horses as a ladder towards progress. Note increase in skul l size, decrease in the number of toes, and increase in the height of the teeth. The skui ls are also arranged in stratigraphic order. W D Matthew used this il lustration in several publications. This version comes from an article in the Quarterly Review of Biology for 1926. NEG NO. 37969. Courtesy Department of Library Services, American Museum of Natu- ral History.

* This parenthetical inspired Roger Angell's letter and led directly to research and writing of the essay preceding

this piece. ,

92 RESONANCE J November 2002

instructors called "size place" (also stratigraphic order in this case). The most familiar of

all illustrations, first drawn early in the century for the American Museum of Natural

History's pamphlet on the evolution of horses, by W D Matthew, but reproduced

hundreds of times since then, shows the whole story: size, toes, and teeth arranged in a row

by order of appearance in the fossil record. To cite just one example of this figure's

influence, George W Hunter reproduced Matthew's chart as the primary illustration of

evolution in his high-school textbook of 1914, A Civic Biology. John Scopes assigned this

book to his classes in Tennessee and was convicted for teaching its chapters on evolution,

as William Jennings Bryan issued his last hurrah (see Essay 28): "No more repulsive

doctrine was ever proclaimed by man ... may heaven defend the youth of our land from

[these] impious babblings."

But what is so wrong with these evolutionary ladders? Surely we can trace an unbroken

continuity from Hyracotherium to modern horses. Yes, but continuity comes in many

more potential modes than the lock step of the ladder. Evolutionary genealogies are

copiously branching bushes - a n d the history of horses is more lush and labyrinthine than

most. To be sure, Hyracotherium is the base of the trunk (as now known), and Equus is the

surviving twig. We can, therefore, draw a pathway of connection from a common

beginning to a lone result. But the lineage of mode.rn horses is a twisted and tortuous

excursion from one branch to another, a path more devious than the road marked by

Ariadne's thread from the Minotaur at the center to the edge of our culture's most famous

labyrinth. Most important, the path proceeds not by continuous transformation but by

lateral stepping (with geological suddenness when punctuated equilibrium applies, as in

this lineage, at least as read by yours truly, who must confess his bias as coauthor of the

theory).

Each lateral step to a new species follows one path among several alternatives. Each

extended lineage becomes a set of decisions at branching points - only one among

hundreds of potential routes through the labyrinth of the bush. There is no central

direction, no preferred exit to this maze - just a series of indirect pathways to every twig

that ever graced the periphery of the bush.

As an example of distortions imposed by converting tortuous paths through bushes into

directed ladders, consider the men associated with the two classical iconographies repro-

duced here. When Huxley made his formal capitulation to Marsh's interpretation in print

(1880), he extended the ladder of horses as a metaphor for all vertebrates. Speaking of

modern reptiles and teleost fishes, Huxley wrote (1880, p.61): "They appear to me to be

RESONANCE I November 2002 93

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The evolution of horses depicted as at least a modest bush by G G Simpson in 1951. NEG.NO. 328907. COURTESY DEPARTMENT OF

LIBRARY SERVICES, AMERI-

CAN MUSEUM OF NATURAL

HISTORY.

off the main line of evolution - to represent, as it were, side tracks starting from certain

points of that line." But teleosts (modern bony fishes) are an enormously successful

group. They stock the world's ocean's, lakes, and rivers and maintain nearly 100 times as

many species as primates (and more than all mammals combined). How can we call them

"off the main line" just because we can trace our own pathway back to a common ancestry

with theirs more than 300 million years ago?

W D Matthew slipped into an equally biased assessment of value because his designation

of one pathway as a ladder forced an interpretation of all others as diversions. Matthew

(1926, p. 164) designated his ladder as the "direct line of succession," but acknowledged

94 RESONANCE I November 2002

that "there are also a number of side branches, more or less closely related." Three pages

later, Matthew adds the opprobrium of near indecency to his previous charge of mere

laterality, as he describes (p. 167) "a number of side branches leading up in a similar

manner to aberrant specialized Equidae now extinct." But in what way are extinct

lineages more specialized than a modern horse or in any sense more peculiar? Their

historical death is the only possible rationale for a designation of aberrancy, but more than

99 percent of all species that ever lived are extinct - and disappearance cannot be the

biological equivalent of a scarlet letter. We might as well call modem horses aberrant

because, much to Montezuma's later sorrow, they became extinct in the land of their birth.

Yet we have recognized the bushiness of horse evolution from the very beginning. How

else did Marsh forestall Huxley but by convincing him that his European "genealogy" of

horses formed a stratigraphic sequence of discontinuous stages, falsely linking several side

branches that had disappeared without issue?

As an example of bushiness, and a plug for the value of appropriate metaphors in general,

consider the finest book on the evolution of horses ever written for popular audiences -

G G Simpson's Horses (1951). Simpson redrew the genealogy of horses as a modest bush

with no preferred main line. He also criticized the conceptual lock, imposed by the bias of

the ladder when he noted that modern one-toed horses are a side branch and extinct three-

toed creatures the main line (if any center can be designated at all).

As nearly as there is a straight line in horse evolution, it culminated and ended with these animals

[the three-toed anchitheres], which, like their ancestors, were multiple-toed browsers. From this

point of view, it is the line leading to modern horses that was the side branch, even though it

outlasted the straighter line of horse evolution [p. 130].

Yet Simpson, who held a lifelong commitment to the predominant role of evolution by

transformational change within populations rather than by accumulation across numer-

ous events of discrete, branching speciation, could not entirely let go of biases, imposed by

the metaphor of the ladder. In one revealing passage, he accepts bushiness, but bemoans

the complexities thus introduced, as though they clouded evolution's essence of transfor-

mational change:

Miohippus ... intergraded with several different descendant groups. It is sad that this introduces

possible confusion into the story, but there is not much point in criticizing nature for something that

happened some millions of years ago. It would also be foolish to try to ignore the complications,

which did occur and which are a very important part of the record.

RESONANCE J November 2002 95

C cY#YC

But these "complications" are not a veil upon the essence of lineal descent; they are the

primary stuff of evolution itself.

Moreover, Simpson restricted his bushiness as much as possible and retained linearity

wherever he could avoid an inference of branching. In particular, he proposes the specific

and testable hypothesis (see his illustration) that the early part of the record - the sequence

of Hyracotherium-Orohippus-Epihippus-Mesohippus-Miohippus-Hypohippus - tells a story

of linear descent, only later interrupted by copious branching among three-toed brows-

ers: "The line from Eohippus to Hypohippus, for example, exemplifies a fairly continuous

phyletic evolution" (p. 217). Simpson especially emphasizes the supposedly gradual and

continuous transformation from Mesohippus to Miohippus near the top of this sequence:

The more progressive horses of the middle Oligocene and all the horses of the late Oligocene are

placed by convention in a separate genus, Miohippus. In fact Mesohippus and Miohippus

intergrade so perfectly and the differences between them are so slight and variable that even experts

find it difficult, at times nearly impossible, to distinguish them clearly.

The enormous expansion of collections since Simpson proposed this hypothesis has

permitted a test by vertebrate paleontologists Don Prothero and Nell Shubin. Their

results falsify Simpson's gradual and linear sequence for the early stages of horse evolution

and introduce extensive bushiness into this last stronghold of the ladder.

Prothero and Shubin have made four major discoveries in the crucial segment of history

that Simpson designated as the strongest case for a gradualistic sequence of lineal

transformation - the transition from Mesohippus to Miohippus.

1. Previous experts were so convinced about the imperceptibly gradual transition between

these two genera that they declared any search for distinguishing characters as vain, and

arbitrarily drew the division between Mesohippus and Miohippus at a stratigraphic bound-

ary. But far richer material available to Prothero and Shubin has permitted the identifica-

tion of characters that cleanly distinguish the two genera. (Teeth are the hardest part of a

vertebrate skeleton and the fossil record of mammals often contains little else. A technical

course in the evolution of mammals is largely an exercise in the identification of teeth, and

an old professional quip holds that mammalian evolution is the interbreeding of two sets

of teeth to produce some slightly modified descendant choppers. Miohippus and Mesohippus

do not have distinctive dentitions, and previous failure to find a clear separation should

not surprise us. The new material is rich in skull and limb bones.) In particular, Prothero

96 RESONANCE I November 2002

and Shubin found that Miohippus develops a distinctive articulation, absent in ancestral

Mesohippus, between the enlarging third metatarsal (the foot bone of the digit that will

become the entire hoof of modern horses) and the cuboid bone of the tarsus (ankle) above.

2. Mesohippus does not turn into Miohippus by insensible degrees of gradual transition.

Rather, Miohippus arises by branching from a Mesohippus stock that continues to survive

long afterward. The two genera overlap in time by at least 4 million years.

3. Each genus is itself a bush of several related species, not a rung on a ladder of progress.

These species often lived and interacted in the same area at the same time (as different

species of zebra do in Africa today). One set of strata in Wyoming, for example, has yielded

three species of Mesohippus and two of Miohippus, all contemporaries.

4. The species of these bushes tend to arise with geological suddenness, and then to persist

with little change for long periods. Evolutionary change occurs at the branch points

themselves and trends are not continuous marches up ladders, but concatenations of

increments achieved at nodes of branching on evolutionary bushes. Of this phenomenon

Prothero and Shubin write:

There is no evidence of long-term changes within these well-defined species [of Mesohippus and

Miohippus] through time. Instead, they are strikingly static through millions of years. Such stasis

is apparent in most Neogene [later] horses as well, and in Hyracotherium. This is contrary to the

widely-held myth about horse species as gradualisticaUy varying parts of a continuum, with no real

distinctions between species. Throughout the history of horses, the species are well-marked and

static over millions of years. At high resolution, the gradualistic picture of horse evolution becomes

a complex bush of overlapping, closely related species.

Bushiness now pervades the entire phylogeny of horses.

We can appreciate this fundamental shift in iconography and meaning, but where is the

"precious irony" that I promised? What is "life's little joke" of my title? Simply this. The

model of the ladder is much more than merely wrong. It never could provide the promised

illustration of evolution progressive and triumphant - for it could only be applied to

unsuccessful lineages.

Bushes represent the proper topology of evolution. Ladders are false abstractions, made by

running a steamroller over a labyrinthine pathway that hops from branch to branch

through a phylogenetic bush. We cannot force a successful bush of evolution into a ladder

RESONANCE I November 2002 97

because we way follow a thousand pathways through the maze of twigs, and we cannot find

a criterion for preferring one route over another. Who ever heard of the evolutionary trend

of rodents or of bats or of antelopes? Yet these are the greatest success stories in the history

of mammals. Our proudest cases do not become our classic illustrations because we can

draw no ladder of progress through a vigorous bush with hundreds of surviving twigs.

But consider the poor horses. Theirs was once a luxuriant bush, yet they barely survive

today. Only one twig (the genus Equus, with horses, zebras, and asses) now carries all the

heritage of a group that once dominated the history of hoofed mammals - and with

fragility at that, for Equus died in the land of its birth and had to be salvaged from a stock

that had migrated elsewhere. (In a larger sense, horses form one of three dwindling lines

- tapirs and rhinos are the others - that now represent all the diversity of the formerly

dominant order Perissodactyla, or odd-toed ungulates, among hoofed mammals. This

mighty group once included the giant titanotheres, the clawed chalicotheres, and

Baluchitherium, the largest land mammal that ever lived. It now hangs on as a remnant in

a world increasingly dominated by the Artiodactyla, or even-toed ungulates - cows, deer,

antelope, camels, hippos, giraffes, pigs, and their relatives.)

This is life's little joke. By imposing the model of the ladder upon the reali~ of bushes,

we have guaranteed that our classic examples of evolutionary progress can only apply to

unsuccessful lineages on the very brink of extermination - for we can linearize a bush only

if it maintains but one surviving twig that we can falsely place at the summit of a ladder.

I need hardly remind everybody that at least one other mammalian lineage, preeminent

among all in our attention and concern, shares with horses the sorry state of reduction

from a formerly luxuriant bush to a single surviving twig - the very property of extreme

tenuousness that perrnits us to build a ladder reaching only to the heart of our own folly

98 RESONANCE ] November 2002